Underground nuclear testing

Nuclear devices are often tested underground to prevent radioactive material released in the explosion reaching the surface and contaminating the environment, and to ensure a degree of secrecy.
The release of radiation from an underground nuclear explosion - an effect known as "venting" - would give away clues to the technical composition and size of a country's device, and therefore its nuclear capability.



PREPARATION
The test site is carefully geologically surveyed to ensure suitability. Such tests usually take place well away from population centres
The nuclear device is placed into a drilled hole or tunnel usually between 200-800 metres below the surface, and several metres wide.

A lead-lined canister containing monitoring equipment is lowered into the shaft above the chamber. The hole is then plugged with gravel, sand, gypsum and other fine materials to contain the explosion and fallout underground.

THE TEST
The device is remotely detonated from a surface control bunker. The nuclear explosion vaporises subterranean rock, creating an underground chamber filled with superheated radioactive gas.


Seismic shockwaves from the explosion were recorded in Japan

As this cools, a pool of molten rock collects at the bottom of the chamber.


Minutes or hours later as pressure falls, the chamber collapses in on itself causing subsidence and a crater to appear on the surface.

North Korea is believed to have conducted a test of a relatively small device at a site called P'unggye-yok in a remote area in the east of the country, near Gilju.

Recent satellite images appear to show a number of buildings and earthworks in keeping with other nuclear test sites.


by : www.news.bbc.co.uk